Presently the Long Term Evolution (LTE) standard accommodates both paired spectrum for Frequency Division Duplex, FDD and unpaired spectrum for Time Division Duplex, TDD operation. LTE FDD using the paired spectrum is anticipated to form the migration path for the current 3G services being used around the globe, most of which use FDD paired spectrum. However there has been an additional emphasis on including TDD LTE using unpaired spectrum. TDD LTE which is also known as TD-LTE is seen as providing the evolution or upgrade path for Time Division-Synchronous Code Division Multiple Access (TD-SCDMA).
One benefit of deploying the LTE TDD system is that it is possible to dynamically change the Uplink (UL) and Downlink (DL) capacity ratio to match demand. More specifically LTE-TDD permits asymmetric UL-DL allocations in a radio frame. Asymmetric resource allocation is realized by providing seven different semi-statically configured UL-DL subframe configurations for a given frame, as being specified in Table 4.2-2 3rd Generation Partnership Project (3GPP) TS 36.211 v10.5.0 (2012 June). These allocations can provide between 40% and 90% DL subframes. Typically the UL-DL configuration in use is signalled to UEs (and changed) via system information provided on the dedicated broadcast channel. Consequently the UL-DL configuration is only semi-statically configured and so may not adapt/match to the instantaneous traffic situation. This is inefficient in term of resource utilization, particularly in cells with a small number of users where the traffic situation typically changes more frequently.
To address the inefficiencies in UL-DL configuration during traffic fluctuations, a flexible TDD configuration has been considered for LTE-Advanced (LTE-A) Release 11. Studies on flexible TDD configurations have revealed significant performance benefits by allowing TDD UL-DL reconfiguration based on traffic adaptation in small cells. The studies also recommend interference mitigation scheme(s) for systems with TDD UL-DL reconfiguration.